How to create an automatic backup for the configuration of the cisco WLC to a TFTP/SFTP server ?

You cannot do an automatic backup of config in WLC 5500 whereas you can schedule a reboot . You can still used the commands below for initiating a config backup

WLC>transfer upload datatype config

WLC>transfer upload mode tftp/sftp

WLC>transfer upload path <location of config file>

WLC>transfer upload serverip <IP of TFTP server>

WLC>transfer upload start

Cisco Prime Infrastructure would be the easiest way to automate the WLC configuration backup at regular interval.